A novel instrument is proposed to noninvasively map internal body temperature during hyperthermia cancer therapy. Previous efforts to treat cancer with heat have been seriously hampered by the lack of such temperature maps, making it difficult and often impossible to maintain the affected part within the therapeutic temperature range while avoidng damage to healthy tissue from excessive heating. The proposed instrument will remedy this situation by providing a means to accurately monitor the application of heat by means of microwaves or other means for hyperthermia cancer therapy. The instrument relies upon the temperature dependent speed of ultrasound in tissue, from which temperature may be reconstructed by computer tomography techniques. Accurate calibration will be assured by including one or more of the latest available invasive point temperature probes within the instrument's field of view. Evaluation of the method will involve laboratory tests on artificial models, phantoms, and excised tissue samples, after which the prototype instrument will be clinically evaluated using the human breast as the initial imaging site due to its relative homogeneity. Further tests will be conducted on a second generation scanner designed for imaging more difficult parts of the body such as the limbs, the neck, and the abdomen, where bone or gas may limit the field of view or cause troublesome reflection or refraction. Diagnostic capabilities inherent in these instruments will also be developed, to aid in the detection of cancer, for the localization and identification of cancer during therapy including the possible development of new malignancies. The imaging capabilities of the second instrument will fill a critical gap in our present ability to obtain diagnostically useful images of cancer susceptible organs in the abdomen such as the liver and pancreas.